1. Field of the Invention
The invention relates to a process for the recovery of monoglycerides and diglycerides from a mixture containing monoglycerides, diglycerides, and triglycerides by extraction in countercurrent with a supercritical extractant.
2. Brief Description of the Background of the Invention Including Prior Art
Monoglycerides are partial esters of glycerol with higher molecular fatty acids. Commercially available monoglycerides consist of mixtures of monoesters and diesters with minor additions of triesters, and they may be obtained by transesterification of triglycerides with glycerol or by reaction of glycerol with fatty acids.
Monoglycerides possess emulsifying, stabilizing, plasticizing and thickening properties. As the monoesters and diesters of glycerol are edible, they are used in various fields of the foodstuff industry, pharmaceutical industry, and in cosmetics. Normally, monoglycerides are water/oil emulsifiers, but by minor additions of, for example soaps, polyethylene oxide compounds, sulfated alcohols, they become "self-emulsifying" and are good oil/water emulsifiers under these conditions. Depending on the additive, they are able to form acid-stable and electrolyte-stable emulsions. The monoglycerides of higher fatty acids, such as, for example, glycerol monostearate, are used as lubricants in the processing of plastic materials. Monoglycerides obtained by molecular distillation and having a monoglyceride content of more than 90% are mainly used in the foodstuff industry, such as in farinaceous products, sweets and baking additives, margarine, ice cream.
By the addition of monoglyceride, up to 5% of palmitic acid/stearic acid-monoglycerides with a purity of 90% or of 10% palmitic acid/stearic acid-mono/diglyceride, self-emulsifying properties of the shortenings intended for baking are achieved (superglycerolated shortenings).
The term "shortening" expressis verbis means to shorten something and is derived from baking properties. Due to the specific structure, monoglycerides are able to change the plasticizing action of starch and gluten in the preparation of dough by entering finely divided into the homogenous plasticized material, thus breaking up the materials and interrupting them to make the dough more smoother, i.e. shorter. At the same time the incorporation of air is facilitated so that, altogether, bakery products are obtained with increased volume and improved "shortness".
The essential components of margarine are edible fats and oils, drinking water, emulsifiers. As emulsifiers, lecithin, the egg yolk and /or monoglycerides and diglycerides of edible fatty acids may be used. Furthermore, the margarine may contain aroma finishing additives, sour milk, buttermilk, skim milk, fermented milk, cooking salt, starch sirup, citric acid and/or other edible acids, vitamins as well as permissible certified preservatives and certified dyestuffs (usually carotine or carotine-containing oils). Emulsifiers are obligatory auxiliary products in the preparation of margarine, because they enable the formation of water in oil emulsions. The most common ones are monoglycerides and plant lecithin assisting each other in the emulsifying action. Used in practice are products having about 40 and about 90%, respectively, of monoglycerides of C.sub.16 /C.sub.18 acids (palmitic acid, stearic acid, also in mixture with oleic acid) which, in addition, contain 60 and 10%, respectively, of diglycerides. Additions of up to 0.5 and 0.25%, respectively, corresponding to about 0.2% of monoglyceride, based on the fatty phase, are common. In the preparation of low-calorie margarine, higher proportions of emulsifier are usually necessary.
The monoglycerides and diglycerides are obtainable by esterification of glycerol with fatty acids. Another route is the transesterification of triglycerides with glycerol. Furthermore, an enzymatic cleavage of triglycerides has lately found acceptance in technology. All methods result in a blend of monoglycerides, diglycerides, and triglycerides. During the esterification, an equilibrium mixture of about 60% of monoglycerides, 35% of diglycerides, and 5% of triglycerides is finally generated. The mixture is separated by molecular distillation. A disproportionation occurs to a minor extent at high temperatures in the film evaporator so that the monoglycerides or diglycerides contain small amounts of the other two esters and minor amounts of free fatty acids.
Because of the disproportionation at the temperatures prevailing in the film evaporator, monoglycerides having contents of more than 95% may not be obtained by molecular distillation at ambient pressure in an economically feasible manner. However, there exists a great interest in monoglycerides possessing a purity of 99% and more.
This is surprisingly achieved by the process according to this invention by means of the extraction with a supercritical extractant consisting of a highly volatile hydrocarbon having 2 to 6 carbon atoms and e.g. carbon dioxide and/or N.sub.2 O.
It is known that monoglycerides may be removed from a mixture of monoglycerides, diglycerides, and triglycerides by means of dense carbon dioxide. However, pressures of more than 350 at. at temperatures of 40.degree. C. are necessary for this. Furthermore, the loading of dense carbon dioxide even at pressures of 350 at is still so small (less than 1%) that an economical recovery of monoglycerides of high purity is not possible.
Also suggested was the use of, for example, acetone as entrainer (DE-OS 2,340,566.5). By this the monoglycerides arrive at the head product during the countercurrent extraction as they are the more soluble components. However, the separation factors are relatively low, so that the recovery of pure monoglycerides is economically not interesting, all the more so, as the achieved loadings of 1 to 1.5% are small. Furthermore, the removal of the entrainer acetone from the product is cumbersome.